I thought it was the Aurora at first - I've never seen the Aurora - but then I realised they were too linear and static.

Turns out there was a snow machine at an artificial ski slope nearby that had been switched on during the cold weather, and had malfunctioned, churning out a haze of microscopic ice particles which settled into a thermal layer in the calm atmosphere of that windless night, creating a sort of mirror that reflected the city lights.

That's a butterfly - moths keep their wings down and flat, butterflies keep them upright like that.

Was tired when I wrote that. More specifically it is a Grass Skipper (thats what a couple people told me and what Google brought up when searching), which is a type of butterfly. However some moths do have upright wings.